As component densities and signal rates increase, it has become of importance to arrange the circuitry into as dense a structure as possible while still retaining ability to conduct repair and maintenance operations. There has developed in the art a structural system wherein the components, such as integrated circuit devices, are mounted on generally insulating planar members with printed wiring interconnecting the component terminals and with terminals along an edge of the planar member. The component and printed wiring bearing planar members have become known in the art by such terms as boards, modules and cards. For consistency, the term card will be used. These cards are both rigid and flexible. The edge of the card, in turn, fits into a socket that makes an electrical interconnection between each terminal along the edge of the card and external circuitry in the overall electronic apparatus so that the cards are parallel to each other and perpendicular to the socket.
Progress in the art is encountering several interrelated limitations. As component density and capability increase, the number of terminals on the edge of the card increases which results in closer spacing. The sockets into which the edges of the card fit must be strong enough to support the card and do it with electrically reliable contact on each terminal which results in socket structural limits on both the number of contacts on a card edge and the closeness of the cards. As component capability increases, it becomes necessary to combine several cards into an overall electrical function and when this is done, fan-out wiring or combinatorial switching or logic circuitry is needed.
There has been some progress in the art on a portion of the limitations. The high density contacting limitation has been addressed through the zero insertion force technique wherein in essence a separate force from the insertion force supplies the contact pressure. One example of such a structure is shown in U.S. Pat. No. 3,992,653 wherein cam actuated lateral pressure is applied after card insertion. Another example is shown in U.S. Pat. No. 4,834,660 wherein the contacts on the card edge extend around a resilient member on the card edge and contact pressure is applied through bolts that pull the contacts and the socket together.